1. Field of the Invention
The present invention relates to an apparatus and method for performing Automatic Retransmission reQuest (ARQ) in a wireless communication system, and in particular, to an apparatus and method for performing ARQ in a wireless communication system using a relay scheme.
2. Description of the Related Art
In a conventional wireless communication system, data communication is achieved through a direct link between a fixed Base Station (BS) and a Mobile Station (MS). However, since the location of the BS is fixed in the wireless communication system, a wireless network architecture has a low flexibility when environments change between the BS and the MS. Thus, there is a demerit in that communication services cannot be effectively provided in a wireless environment where traffic distribution and call demands are rapidly changed.
To address this shortcoming, the aforementioned conventional cellular wireless communication system may employ a data transmission method based on a multi-hop relay scheme by using a fixed or movable Relay Station (RS) and a general MS. In the wireless communication system based on the multi-hop relay scheme, a network can be rapidly reconfigured in response to a change in the surrounding environment, and the entire wireless network can be further effectively managed. For example, the wireless communication system based on the multi-hop relay method can broaden a cell service coverage and increase a system capacity. When a channel quality is poor between a BS and an MS, an RS may be installed between the BS and the MS so that a multi-hop relay path is formed via the RS. By doing so, a wireless channel having a better channel quality can be provided to the MS. Moreover, by using the multi-hop relay scheme, the BS can provide a high-speed data channel in a cell boundary region having a poor channel quality and also can extend the cell service coverage.
According to a channel quality in a wireless segment in the wireless data communication, errors may occur in specific data. An error control/recovery technique may use an Automatic Retransmission reQuest (ARQ) scheme or a Frame Error Check (FEC) scheme. In the ARQ scheme, erroneous data is retransmitted by a transmitting end at the request of a receiving end. In the FEC scheme, the erroneous data is corrected.
When the ARQ scheme is used, a result obtained by checking packet errors, for example, using Cyclic Redundancy Check (CRC), has to be fed back from a receiving end to a transmitting end. When a packet is initially transmitted from the transmitting end, the receiving end decodes the received packet. In this case, if no error is detected, an ACK signal is transmitted to the transmitting end, and otherwise, a NACK signal is transmitted to the transmitting end. In response to the ACK/NACK signals received from the receiving end, the transmitting end may retransmit a previous packet or transmit a new packet. In this case, the transmitting end transmits a new packet upon receiving the ACK signal, and transmits a previous packet upon receiving the NACK signal.
For reliable data transmission, the wireless communication system generally employs two retransmission methods. One is a ‘Media Access Control (MAC) ARQ’ operating in a MAC layer, and the other is a ‘Hybrid ARQ (HARQ)’ operating in a Physical (PHY) layer. The conventional MAC ARQ scheme will now be described.
FIG. 1 illustrates a signal exchange process when ARQ is performed in a wireless communication system using a conventional analog RS. The conventional RS will hereinafter be referred to as a repeater.
The conventional wireless communication system is constructed of at least one BS, at least one repeater, and at least one MS. For convenience of description, one BS 10, one repeater 11, and one MS 12 are depicted in FIG. 1. It will be assumed that the BS 10 transmits to the MS 12 two MAC Protocol Data Units (hereinafter, MAC PDUs), and the MS 12 receives data via the repeater 11.
In FIG. 1, in step 101, the BS 10 generates one PHY layer data (or PHY DATA) 1 using two MAC PDUs and transmits it to the repeater 11. Each MAC PDU, that is, a data transmission unit of a MAC layer, includes a MAC header, a payload containing actual data, and an error check code (e.g., CRC code) for detecting errors of the payload. The PHY DATA 1 includes at least one MAC PDU.
In step 103, the repeater 11 simply amplifies the amplitude of a signal containing the PHY DATA 1 received from the BS 10 and relays the amplified signal (indicated by PHY DATA 2 in the figure) to the MS 12.
In step 105, the MS 12 separates MAC PDUs from the PHY DATA 1 received from the repeater 11 and checks errors of each MAC PDU. The error checking is performed using the CRC code included in the MAC PDU. The MS 12 generates an error check code by using the payload of the received MAC PDU and compares the generated error check code with the received error check code, thereby detecting errors. It will be assumed that errors are detected from the MAC PDU 2 among the separated MAC PDUs.
In the case where the MAC PDU 1 is errorless data and the MAC PDU 2 is erroneous data, in step 107, the MS 12 transmits a control-message so that an Acknowledgment (ACK) response is provided for the MAC PDU 1, and a Negative-Acknowledgment (NACK) response is provided for the MAC PDU 2. In step 109, the repeater 11 amplifies again the amplitude of a signal containing the control-message received from the MS 12 and then relays it to the BS 10.
Also, in step 109, the BS 10 analyzes the control-message received from the repeater 11 and determines which MAC PDU is requested to be retransmitted. Herein, the MAC PDU 2 is determined to be retransmitted. In step 111, the BS 10 generates PHY retransmission data by using the MAC PDU 2 requested to be retransmitted, and then transmits it to the repeater 11. In step 113, the repeater 11 simply amplifies the amplitude of a signal containing the PHY retransmission data received from the BS 11 and relays it to the MS 12. Accordingly, the MS 12 requests the BS 11 to retransmit erroneous data via the repeater 11, and then receives the retransmitted data via the repeater 11.
As described above, the conventional repeater simply amplifies the amplitude of an analog signal received from a BS and then relays it to an MS. However, such repeater simply performing a relay function is not efficient considering that an intelligent RS is expected to be used in the near future. The intelligent RS can independently make a decision and utilize resources rather than simply relaying a signal received from the BS. In other words, in the future, unlike the convention repeater, an RS operating both in a MAC layer and a PHY layer will be used. Since the RS can analyze messages exchanged between a BS and an MS, it is more effective than the convention relay scheme in which a transmission signal is simply amplified. Accordingly, there is a need for an ARQ operation method in consideration of such intelligent RS.